CI: Ignore version duplicates on publication to TestPyPI

This commit prevents the failure of the GitHub Actions job "Publish
distribution package to TestPyPI" on most pushes (see discussion at
#721 (comment)),
since recent commit a3e1325 "CI: Publish PyPI releases using GitHub
Actions workflows".

Indeed, TestPyPI doesn't allow repushing the same version. Use
`skip-existing` to prevent to such failures on the CI.

An alternative would be to use the `setuptools-scm` plugin, but it
requires more changes (to define the version in `pyproject.toml`, to
fetch tags from GitHub in `publish.yaml`, and to exclude `.*` +
`.github/**` files in `MANIFEST.in`). Moreover it doesn't work as is: to
prevent errors like `HTTP 400 The use of local versions in
'1.36.2.dev2+gd4f1c14' we would need `local_scheme =
"no-local-version"`, which would push duplicated version on TestPyPI
anyway.

Author: adrienverge

.github/workflows/publish.yaml

@@ -46,6 +46,7 @@ jobs:
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
           repository-url: https://test.pypi.org/legacy/
+          skip-existing: true
 
   publish-to-pypi:
     name: Publish distribution package to PyPI